How to Make It
Preheat the oven to 425°. On a large rimmed baking sheet, toss the leeks and potatoes with 2 tablespoons of olive oil; season with salt and pepper. Arrange the leeks and potatoes on the baking sheet, cut side down, and roast them on the bottom rack of the oven for 20 minutes, or until the leeks are lightly browned and tender. Transfer the leeks to a work surface and cut them crosswise into 2-inch lengths. Continue to roast the potatoes for about 10 minutes longer, or until they are browned and tender.
In a small bowl, whisk the remaining 1/4 cup of olive oil with the red wine vinegar, mustard and capers and season the vinaigrette with salt and pepper. In a medium bowl, toss the greens with 1 tablespoon of the vinaigrette and arrange on 4 plates. Put the leeks and potatoes in the bowl and toss with all but 2 tablespoons of the vinaigrette. Arrange the leeks and potatoes on the greens. Garnish the salads with the eggs, drizzle the remaining vinaigrette on top and serve.